This invention relates generally to xerographic copiers, and more particularly to an improved applicator for depositing triboelectrically-charged toner particles onto an electrostatically-charged latent image to render visible and develop this image.
In the xerographic technique, a photoconductive insulating layer whose surface is uniformly charged electrically is first exposed to an illuminated pattern of light and shadow of the intelligence to be recorded. The blanket charge on the layer is selectively dissipated by the illuminated pattern to yield a latent electrostatic image. Thereafter, to develop the image, finely-divided pigmented thermoplastic powder or toner is deposited on the latent image, the toner particles adhering to the electrostatically-charged areas in proportion to the charges thereon.
In a plain paper xerographic printer such as that disclosed in the Boulton U.S. Pat. No. 2,987,037, the photoconductive insulating layer is supported on a rotating drum and the toner image developed on the surface of this layer is transferred from the drum onto a sheet of ordinary paper. The developed image on the paper is then fixed thereto by heat which fuses the toner particles to the paper. It is also known to effect such fusion by the application of pressure rather than heat.
In a treated-paper xerographic printer such as that disclosed in the Shelffo U.S. Pat. No. 3,520,604, there is no need to transfer the developed toner image from the photoconductive insulating layer, for in this instance use is made of paper coated with photoconductive zinc oxide particles dispersed in a film-forming resin binder. The coated surface of the paper is subjected to a blanket electrostatic charge which is then exposed to the light pattern to be recorded to create a latent image thereon. This latent image is developed by toner which is directly fixed onto the treated paper, thereby obviating the transfer step characteristic of an untreated paper printer.
The present invention is concerned primarily with apparatus adapted to deposit toner onto an electrostatically-charged latent image formed on a photoconductive layer, the invention being fully applicable both to treated and plain paper xerographic printers.
Among the known types of toner development systems are cascade, brush and magnetic brush development as well as powder cloud and liquid development. A cascade development system, such as that disclosed in the Shaffert U.S. Pat. No. 2,576,047, is adequate for making ordinary line copies, but it has limited value where solid area development is required. For solid area development producing a good copy of an original which includes photographs as well as printed text, brush applicators have greater utility.
In a brush development system such as the one described in Boulton U.S. Pat. No. 2,987,037, use is made of a soft fur brush which acts not only to pick up and transfer toner particles onto a latent electrostatic image, but also to impart a triboelectric charge to particles actually brought into contact with the brush hairs.
Currently, a widely used type of toner applicator in commercially-available xerographic printers is the magnetic brush system, such as that disclosed in the Morse U.S. Pat. No. 3,674,532. A magnetic brush developer requires a developer mix constituted by ferromagnetic iron particles intermingled with toner powder. This mix is picked up by a magnet having a bristle-like or brush formation, the iron particles being magnetically attracted to the bristles and the toner particles adhering electrostatically to the iron particles. The bristles are electrically conductive and contribute to the transfer of toner to the electrostatically-charged latent image.
A xerographic printer including a magnetic brush development system involves relatively costly and complex apparatus and is subject with prolonged use to contamination and other problems. Moreover, the iron particles are physically worked in the course of operation and the properties thereof are degraded, making it necessary from time to time to shut down the machine to replace the iron particles in the mix.
In order to overcome the drawbacks of existing magnetic brush development systems, it is known to provide toner particles having a ferromagnetic core covered with a thermoplastic resin sheath, thereby avoiding the need for a mix. But such two-component particles are expensive and add substantially to the cost of making copies.